The present invention concerns a new powder composition and a process for the preparation thereof. More specifically, the invention concerns a new hydrogen storage alloy powder composition mainly for use in rechargeable metal hydride electrodes as for example in nickel metal hydride (NiMH) rechargeable batteries. Additionally the powder compositions will improve the properties of hydrogen storage alloys used for gaseous storage of hydrogen.
With the popularisation of portable appliances demands for small size rechargeable batteries are increasing rapidly. Presently, the main small size rechargeable battery, the Ni--Cd battery, is meeting its limits for the requirements as compact battery. In the future also new requirements for rechargeable batteries for electrical vehicles (EV) will make it necessary to develop new rechargeable batteries. The Ni--Cd battery also has problems of environmental pollution by Cd. The Ni--Cd battery will therefore soon be replaced by the larger capacity NiMH battery, which also contains lower levels of pollutants. The NiMH battery is composed of nickel hydroxide (positive electrode) and a hydrogen storage alloy (negative electrode) and alloy manufacturers are required to develop high performance hydrogen storage alloys.
The article by Koichi Koshiro et al in The Sumitomo Search No 52, March 1995 describes a gas atomisation process (GAP) for high performance hydrogen storage alloys. According to this process raw materials are melted in a vacuum or inert gas atmosphere by induction melting. The molten alloy that falls through the tundish is atomised by inert gas, such as argon, and the atomised powder is collected at the bottom of the chamber. This powder is spherical and intended for use in negative electrodes of NiMH rechargeable batteries.
The advantage using GAP instead of conventional casting techniques is the fine microstructure, elimination of macro-segregation and limited micro-segregation in the gas atomised powder due to the rapid solidification (RS). Another feature is the possibility to develop new alloys especially designed to utilise in the RS process.
Also EP 0 420 669 discloses a gas atomisation process for the preparation of spherical hydrogen absorbing alloy particles for nickel hydride secondary cells.
According to EP 0 647 973 a gas atomised hydrogen absorbing alloy powder including spherical particles might be mixed with mechanically pulverised particles of the same alloy powder. The amount of mechanically pulverised particles in this known powder mixture is at most 80% by weight of the total mixture.
EP 0 588 310 discloses a hydrogen absorbing alloy having a certain type of columnar structure obtained by quenching a molten alloy at a rate of 1800.degree. C./s or higher. This very high quenching rate is achieved by using a single roll method. Although no specific examples are given the patent application also indicates, that the alloy might be prepared by gas atomisation. However, it is not disclosed if and how quenching rates of at least 1800.degree. C./s or higher can be obtained in combination with gas atomisation. The quenched powder known from this application is also heat treated at a temperature of at most 500.degree. C.